Stable isotopic evidence for the excess leaching of unprocessed atmosphericnitrate from forested catchments under high nitrogen saturation

Owing to the elevated loading of nitrogen through atmospheric deposition, some forested ecosystems become nitrogen saturated, from which elevated levels of nitrate are exported. The average concentration of stream nitrate eluted from upstream and downstream of the Kasuya Research forested catchments (FK1 and FK2 catchments) in Japan were more than 90 mu M, implying that these forested catchments were under nitrogen saturation. To verify that these forested catchments were under the nitrogen saturation, we determined the export flux of unprocessed atmospheric nitrate relative to the entire deposition flux (M-atm / D(atm )ratio) in these catchments; because the M-atm / D-atm ratio has re-cently been proposed as a reliable index to evaluate nitro-gen saturation in forested catchments. Specifically, we determined the temporal variation in the concentrations and stable isotopic compositions, including delta O-17, of stream nitrate in the FK catchments for more than 2 years. In addition, for comparison, the same parameters were also monitored in the Shiiba Research forested catchment (MY catchment) in Japan during the same period, where the average stream nitrate concentration was low, less than 10 mu M. While showing the average nitrate concentrations of 109.5, 90.9, and 7.3 mu M in FK1, FK2, and MY, respectively, the catchments showed average delta O-17 values of +2.6 %o, +1.5 %o, and +0.6 %o in FK1, FK2, and MY, respectively. Thus, the average concentration of unprocessed atmospheric nitrate ([NO3atm- ]) was estimated to be 10.8, 5.1, and 0.2 mu M in FK1, FK2, and MY, respectively, and the M-atm / D-atm ratio was estimated to be 14.1 %, 6.6 %, and 1.3 % in FK1, FK2, and MY, respectively. The estimated M-atm / D-atm ratio in FK1 (14.1 %) was the highest ever reported from temperate forested catch-ments monitored for more than 1 year. Thus, we concluded that nitrogen saturation was responsible for the enrichment of stream nitrate in the FK catchments, together with the elevated NO3atm- leaching from the catchments. While the stream nitrate concentration ([NO3- ]) can be affected by the amount of precipitation, the M-atm / D-atm ratio is independent of the amount of precipitation; thus, the M-atm / D-atm ratio can be used as a robust index for evaluating nitrogen saturation in forested catchments.